Pension Plans and Other Postretirement Benefits - Schedule of Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	$ 22.5	$ 19.3	$ 20.2
Interest cost on projected benefit obligation	22.6	28.2	27.4
Expected return on plan assets	(35.0)	(36.3)	(37.0)
Amortization of prior service (credit) cost	(2.5)	(1.1)	(0.9)
Amortization of actuarial (gain) loss	14.4	9.3	10.9
Loss on lump-sum settlements and curtailments	31.6	0.0	18.6
Subtotal	53.6	19.4	39.2
Regulatory adjustment	6.6	39.6	37.4
Net pension cost	60.2	59.0	76.6
Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	5.9	7.4	9.4
Interest cost on projected benefit obligation	6.3	9.0	8.6
Expected return on plan assets	(16.7)	(16.2)	(13.9)
Amortization of prior service (credit) cost	(0.5)	(0.1)	(0.1)
Amortization of actuarial (gain) loss	(2.0)	(0.5)	0.8
Subtotal	(7.0)	(0.4)	4.8
Regulatory adjustment	16.0	10.0	2.2
Net pension cost	9.0	9.6	7.0
Spire Missouri | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	15.7	12.4	12.7
Interest cost on projected benefit obligation	15.8	19.8	19.5
Expected return on plan assets	(24.6)	(25.5)	(27.2)
Amortization of prior service (credit) cost	0.1	0.9	0.9
Amortization of actuarial (gain) loss	11.3	8.7	9.4
Loss on lump-sum settlements and curtailments	26.6	0.0	16.1
Subtotal	44.9	16.3	31.4
Regulatory adjustment	3.9	31.8	32.1
Net pension cost	48.8	48.1	63.5
Spire Missouri | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	5.3	6.8	9.0
Interest cost on projected benefit obligation	4.7	6.9	6.9
Expected return on plan assets	(11.4)	(11.1)	(9.6)
Amortization of prior service (credit) cost	(0.2)	0.3	0.3
Amortization of actuarial (gain) loss	(2.0)	(0.5)	0.9
Subtotal	(3.6)	2.4	7.5
Regulatory adjustment	17.7	11.7	3.9
Net pension cost	14.1	14.1	11.4
Spire Alabama | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	6.1	6.2	6.4
Interest cost on projected benefit obligation	4.9	6.0	5.5
Expected return on plan assets	(6.9)	(7.3)	(6.5)
Amortization of prior service (credit) cost	(2.4)	(1.8)	(1.8)
Amortization of actuarial (gain) loss	3.1	0.8	1.5
Loss on lump-sum settlements and curtailments	5.0	0.0	2.4
Subtotal	9.8	3.9	7.5
Regulatory adjustment	1.8	6.9	4.5
Net pension cost	11.6	10.8	12.0
Spire Alabama | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	0.4	0.4	0.2
Interest cost on projected benefit obligation	1.4	1.8	1.5
Expected return on plan assets	(5.0)	(4.8)	(4.1)
Amortization of prior service (credit) cost	(0.3)	(0.4)	(0.4)
Amortization of actuarial (gain) loss	0.0	0.0	(0.1)
Subtotal	(3.5)	(3.0)	(2.9)
Regulatory adjustment	(1.8)	(1.8)	(1.8)
Net pension cost	$ (5.3)	$ (4.8)	$ (4.7)